The present invention relates to a signal coupling and branching system with which it is possible to establish communications among more than three signal transmitter-receivers through a signal coupling and branching unit. More particularly, the present invention relates to an optical fiber signal coupling and branching unit which is applicable to optical fiber submarine cable networks.
In cases when it has been necessary to establish communications among N-numbered (N.gtoreq.3) signal transmitter-receivers (hereinafter called "nodes"), it has been customary to establish a communication network among them by setting transmissions lines among the nodes, thereby transmitting and receiving information. One example this type of communication network is shown in FIG. 13. FIG. 13 shows a case in which N=4 and a communication network is formed by connecting nodes N1, N2, N3 and N4 through transmission lines C11, C12, C13, C14, C15 and C16. One type of communication network is a star-like network having a structure such that a signal coupling and branching unit to couple and to branch off signals from the nodes is set up at a point other than the N-numbered nodes and the nodes and the signal coupling and branching unit are connected through transmission lines. FIG. 14 shows a case in which N=4 and signals from N1, N2, N3 and N4 to other nodes are only collected through cables C17, C18, C19 and C20, in a signal coupling and branching unit SB. The signals are re-composed in the unit SB with respect to their destinations and transmitted to the nodes, respectively. If a star-like network, having the signal coupling and branching unit, is applied to a transmission system, like a submarine cable transmission system in which the cost of the cable with respect to the installation cost of the system is high, a substantial economic benefit will be realized because the overall length of the cable is shortened. Further, in a submarine cable system, the value of N is not so high, and when the number N is around 3-4, it is economical to transmit information at a high-speed using time-division multiplexing. Apart from the above, it is also possible to use a system in which transmission lines (e.q., fibers) are assigned among nodes. These fibers are incorporated into a transmission cable. Since the number of fibers and repeater circuits increases proportionally, use of such a system is not always more economical than use of time division multiplexing systems.
Further, to perform time-division multiplexing of information signals at each node, there are both word unit multiplexing systems and bit-multiplexing (bit-interleaving) systems. The word unit multiplexing system is ordinarily used in an encoding terminal station. It requires memory for rearranging signals to word structures. On the other hand, the bit-interleaving system does not require this kind of memory and it is sufficient for it to arrange a lower level group of pulse trains in sequence by time division so that it is often used for a higher rate multiplexing unit. In addition, bit-interleaving signals can be exchanged in a simple manner, and when they are applied to transmission systems with many repeaters, it is possible to extract clock signal components even when no communication is ON at some node so that it is advantageous a point of retiming and is suitable for a transmission system requiring simple underwater equipment such as a submarine cable transmission system.
Accordingly, a network system in which information signals multiplexed through a bit-multiplexing system are transmitted from each node and after being re-composed by time division at a signal coupling and branching unit, returned to each node while each node and the unit are connected by N-numbered cables in the form of a star. In addition, as one embodiment of the present invention, a case in which N=3 will also be described hereafter as an example.
To begin with, a network structure in the case of N=3 is shown in FIG. 15. According to this structure, information signals to nodes A, B and C are bit-interleaved and transmitted from a transmitting terminal station.
FIG. 16 shows conditions in which PCM signals to nodes B and C are bit-interleaved at the node A wherein (a) and (b) indicate a train of pulses to the node B and C, respectively, each showing that the n-numbered PCM signal pulses in a bit period T constitute a single frame, with Tf indicating a frame period. If this train of pulses is multiplexed in a bit period T/2, a bit-interleaved train of signal pulses shown by (c) is obtained. The same thing can be said at nodes B and C. Thus the trains of pulses multiplexed at each node are transmitted to the signal coupling and branching unit SB through fibers F incorporated into cables CBL and repeaters R.
Next, in signal coupling and branching unit SB, the connections among the nodes are time divisionally changed by a period (T/2) through switches arranged therein and the signals from each node are re-arranged for their destinations. The operation of the SB is as follows: Assuming that the phase relationships among the trains of pulses from each node to the unit SB are in the conditions shown in FIG. 17(a), the connecting conditions of the switches are controlled repeatedly by the period (T/2) as shown in FIG. 17(b) so that the signals are time-divisionally connected and re-arranged as shown in FIG. 17(c) to be then transmitted to each node through fibers F' and repeaters R' shown in FIG. 15. Then the bit-interleaved signals are received by a receiving terminal station RT at each node and separated to recover the lower-speed signal for each node.
We have described the operation principle of the transmission branching system in which information signals to address are multiplexed by bit at nodes and the multiplexed signals are re-arranged by time division and returned to the nodes.
However, the following problems are encountered when this system is applied to optical fiber submarine cable transmission systems.
In order to explain the operation principle of the transmission branching system briefly, we have taken up, by way of example, a case in which information signals from each node pass on the switches in the unit in synchronism with the connecting conditions of the switch as shown in FIG. 17. However, if the phase relationships between signal and connecting condition of the switch should be differentiated from those shown in FIG. 17(a) and (b), the quality of the signals would deteriorate and the signals would sometimes be connected to the uncorrect nodes. Therefore, to make the connecting conditions of the switch synchronized with the information signals, various kinds of synchronization systems are employed. One example of such a system is one in which one node acts as a reference station and transmits switch control signals while the other node adjusts the transmission timing of the information signals to synchronize with the connecting conditions of switches. The above adjustment is performed every time the system is started and the cable transmission delay time varies so that constant measurements of the cable transmission delay time and the related complicated operations among the nodes become necessary. Furthermore, redundant structures allowing control signals to be transmitted from nodes other than the reference station and monitoring and switching control system are also necessitated.
Another type of system is one in which a memory is provided within the signal coupling and branching unit to store input information signals and the signals addressed to each node are read out time divisionally to branch off them. Still another example of system is one in which the signal coupling and branching unit has a switch control signal source and the phase difference between the signals from each node and the switch connecting conditions is detected to send a transmission timing signal to each node. However, these systems are unsuitable for underwater equipment because they are unavoidably large in size.
As described above, the conventional synchronization system has had the above-mentioned disadvantages of necessitating the complicated co-related operations among the nodes for synchronization and the auxiliary redundant structure for transmitting the switch control signals and the monitoring and switching control system therefore, and also the disadvantage of the circuit of the signal coupling and branching unit being too large for underwater equipment.